The present invention relates to a difference dispersive double-path monochromator which measures the wavelength of incoming light after incidence on a diffraction grating before and after reflection by a reflecting mirror.
FIG. 1 is a diagram schematically showing the basic concept of a conventional difference dispersive double-path monochromator.
For example, incoming light 11 from an optical fiber 10 is collimated by a parabolic mirror 12 and the collimated light beam 13 strikes on a diffraction grating 14. The collimated light beam 13 thus incident on the diffraction grating 14 is diverged into rays of light 15, which are reflected from the grating 14 at different angles according to wavelength. The rays of light 15 impinge on a second parabolic mirror 16, by which they are converged and reflected as light 17 to a plane reflecting mirror 18. Incidentally, since it is necessary that the optical fiber 10 for introducing the incoming light and a photodetector 19 for receiving outgoing light which returns after having impinged on the diffraction grating 14 twice be physically spaced apart in the Z-axis direction, that is, in the direction of grooves of the diffraction grating 14, the optical axis of the return path is adjusted accordingly. On this account, the light 17 incident on the reflecting mirror 18 is reflected in the Z-axis direction and then reflected by a plane reflecting mirror 21 back to the second parabolic mirror 16 in parallel with the reflected light 17 as indicated by 22. The light beams 17 and 22 are spaced, for example, about 5 mm apart. The focus of the second parabolic mirror 16 is positioned at a point intermediate between reflection points of the reflecting mirrors 18 and 21. A slit 23 is provided at a point intermediate between the reflecting mirrors 18 and 21 to limit the beam diameter of the light which is reflected from the former to the latter. The width of the slit 23 is variable in accordance with the bandwidth of the wavelength to be measured.
The light 22 reflected back to the second parabolic mirror 16 is reflected as a collimated light beam 24 back to the diffraction grating 14; rays of light 24 reflected from the grating 14 are again reflected by the first parabolic mirror 12, then the reflected light 26 therefrom is converged and the component of the wavelength desired to measure is allowed to pass through a slit 27 for incidence on the photodetector 19 for conversion to an electric signal.
With the above structure in which the return path from the reflecting mirror 21 to the photodetector 19 via the diffraction grating 14 is shifted in the Z-axis direction with respect to the path of the incident light 11 from the optical fiber 10 to the reflecting mirror 18 via the diffraction grating 14, even if the incident light 15 to the second parabolic mirror 16 from the diffraction grating 14 and the reflected light 17 stays in a plane perpendicular to the axis of rotation 29 of the diffraction grating 14 (which plane will hereinafter referred to as a main cross-sectional profile), the point of incidence of the light 22 to the second parabolic mirror 16 from the reflecting mirror 21 is displaced in the Z-axis direction relative to the point of incidence of the light 15 to the second parabolic mirror 16 from the diffraction grating 14, and consequently, the reflected light 24 from the parabolic mirror 16 goes out of the above-mentioned main cross-sectional profile and strikes on the diffraction grating 14 at a certain angle .theta. to the main cross-sectional profile 30. That is, the angle of incidence on the diffraction grating 14 on the return path is the angle .theta. to a plane 30 perpendicular to the grating grooves of the diffraction grating 14 as shown in FIG. 2A; hence, the relationship between the angles of incidence and reflection from the diffraction grating 14 differ with the paths to and from the reflecting mirrors 18 and 23. In other words, the wavelength .lambda. in the diffraction grating 14, the order m of the diffracted light, the grating groove spacing d, the incidence angle .alpha. and the reflection angle .beta. bear the following relationship when the light 13 is incident in the plane (the main cross-sectional profile) 30 perpendicular to the grating grooves of the diffraction grating 14 as shown in FIG. 2A. EQU m.lambda.=d(sin.alpha.+sin.beta.) (1)
This relationship is used to measure the wavelength of the incoming light. When light is incident on the diffraction grating 14 at the angle .theta. to the main cross-sectional profile 30 as shown in FIG. 2B, the relationship of Eq. (1) becomes as follows: EQU m.lambda.=d(cos.theta.sin.alpha.+cos.theta.sin.beta.) (2 )
Thus, when sweeping the wavelength to be measured by turning the diffraction grating 14 about the axis 29 parallel to the grating grooves, the direction of the reflected light 26 from the first parabolic mirror 12 changes with wavelength as indicated by an angle .phi.. To comply with this, the conventional double-path monochromator has, as disclosed in U.S. Pat. application No. 788,444 (filed Nov. 6, 1991, now U.S. Pat. No. 5,233,405, or Japanese Pat. Laid Application Open Gazette No. 212025/92), for instance, a drive mechanism 31 which moves the slit 27 and the photodetector 19 in ganged relation to the turning of the diffraction grating 14 to always hold the photodetector 19 at the position of the imaging point of the reflected light 26 which differs with wavelength. The drive mechanism 31 is commonly referred to as a tracking mechanism and is required to bring the photodetector 19 to a position precisely corresponding to the rotational position of the diffraction grating 14.
A description will be given further of the problem that the imaging point of the reflected light 26 on the return path shifts in accordance with the wavelength to be measured. Letting the angle of incidence of forward light on the diffraction grating 14, The angle of reflection therefrom. The angle of incidence of backward light on the diffraction grating 14 and the angle of reflection therefrom be represented by .alpha..sub.f, .beta..sub.f, .beta..sub.b and .alpha..sub.b, respectively, Eqs. (1) and (2) hold true for the forward light and backward light and they bear the relationships expressed as follows: EQU m.lambda.=d(sin.alpha..sub.f +sin.beta..sub.f) (3) EQU m.lambda.=d cos.theta.(sin.alpha..sub.b +sin.beta..sub.b) (4)
When the angle of reflection .beta..sub.f of the forward light and the angle of incidence .beta..sub.b of the backward light are the same, setting sin.beta..sub.f =sing.beta..sub.b =K, the difference between the angle of incidence .alpha..sub.f on the diffraction grating 14 on the forward path and the angle of reflection .alpha..sub.b from the diffraction grating 14 on the backward or return path is such as given by Eq. (5). ##EQU1##
Eq. (5) contains elements of the wavelength .lambda. this means that the imaging point of the reflected light 26 moves in accordance with wavelength. On account of this movement of the imaging point, it is necessary in the prior art to bring the photodetector to the position corresponding to the wavelength to be measured, by the aforementioned tracking mechanism 31 or the like. Moreover, the position to which the photodetector 19 is moved by the tracking mechanism 31 must be predetermined for each wavelength; it is therefore necessary, for each equipment, to detect the imaging point of the reflected light 26 for a reference wavelength and prestore the detected value in a memory table or the like.
It is therefore an object of the present invention to provide a difference dispersive double-path monochromator which has an optical configuration that eliminates the movement itself of the imaging point on the return path and consequently precludes the necessity of the tracking mechanism, and hence permits reduction of the manufacturing costs of the monochromator itself and always provides high accuracy.